The present invention relates to a method and to an apparatus for the thermal treatment of substrates, especially semiconductor wafers.
Computer chips, as well as other electronic components, are produced on semiconductor disks, so-called wafers, which are subjected to thermal processes during the manufacturing sequence. These thermal processes require a defined temperature profile of the wafer at a defined atmosphere, in vacuum or defined underpressure or over pressure.
For the temperature treatment of wafers, rapid heating units, also known as RTP units (Rapid Thermal Processing units), are being emphasized more and more. With these units a rapid and well-defined thermal treatment of wafers under prescribed process conditions is possible. RTP units permit a heating of the wafer that is to be treated, as a function of the wafer material, up to 1700° C. and more within a few seconds. A controlled or regulated heating up of the wafer pursuant to prescribed temperature-time curves at heating rates of up to 300° C./s can be achieved with today's units for silicon wafers having a diameter of 300 mm. Higher heating rates of up to 500° C./s can be achieved in an open-loop operation, or with wafers having smaller diameters. RTP units are used in particular for the manufacture of dielectric layers (e.g. SiO2 layer which is produced by oxidation on a silicon wafer, silicon nitride layers, silicon oxynitride layers), implant-annealing processes (for the activation of foreign atoms in the semiconductor wafer), processes for the annealing of dielectric layers, processes for the formation of ohmic contacts, flash-annealing processes (e.g. for the activation of flat doped zones), siliciding processes (e.g. Ti—Co—Ni-silicide), BPSG-reflow processes or processes with which selective reactions are effected in the surface region of the wafer, such as selective oxidation of a gate-dielectric that is disposed below a metal layer, just to name a few processes. Furthermore, by means of modern RTP units, the spatial distribution of foreign atoms, of vacancies, of oxygen and oxygen precipitates can be influenced in a precise manner. A significant advantage of RTP units is that generally, due to the shortened process time during the processing of the wafers, the wafers are individually processed, whereby each wafer undergoes the same process with very high reproducibility. This advantageously reduces the thermal stress of the wafer. Furthermore, due to the possibility of the rapid heating and the rapid cooling off, the RTP units provide for the production of new wafer or component characteristics that could not be achieved with conventional furnace processes.
In order to be able to subject a substrate, such as a semiconductor wafer of silicon, to temperature changes of up to several hundred degrees per second, the wafer is heated in a rapid heating unit, such as is known from DE-A-199 05 524, which originates with the applicant, with radiation from lamps, preferably halogen lamps. The known rapid heating unit has a process chamber (preferably of quartz glass) that is essentially transparent for the lamp radiation and serves for accommodating a substrate. Disposed above and below the process chamber are heat lamps that produce electromagnetic radiation for the thermal treatment of the substrate. The heat lamps and the process chamber can be surrounded by a further chamber (reflector chamber) that can have reflective inner walls in order to reflect the electromagnetic radiation produced by the heat lamps.
A process chamber made of quartz glass is essentially transparent for the spectrum of the electromagnetic radiation that is produced by the heat lamps. The process chamber has inlets and outlets for process gases by means of which a suitable gas atmosphere can be produced within the process chamber during the thermal treatment of the substrate. With suitable dimensioning of the process chamber, it is also possible to produce an underpressure or an overpressure in the chamber.
To measure the wafer temperature, radiation detectors, such as pyrometers, are preferably provided that measure the thermal radiation of the wafer. From the measured thermal radiation of the wafer, it is possible to draw a conclusion with regard to the temperature thereof. To differentiate among the radiation emitted from the wafer, as well as radiation reflected on the wafer and radiation passing through the wafer, the radiation of the heat lamps is modulated. As a result of this modulation, the radiation emitted from the substrate can be differentiated from the radiation reflected at the substrate and the radiation of the heat lamps that passes through. Furthermore, due to the modulation, the reflectivity and transmissivity, and from there the emissivity, of the wafer can be determined, which for a temperature measurement of the wafer is necessary due to the radiation being emitted therefrom. Details of the modulation and of the temperature determination process can be obtained from the aforementioned DE-A-199 05 524 or from U.S. Pat. No. 5,154,512.
However, the temperature measurement based on pyrometers has the problem that there is present in the process or reflector chamber a strong radiation field that makes a differentiation of the radiation emitted from the wafer from the background radiation emitted from the heat lamps difficult. The temperature radiation that is emitted from the wafer and is to be measured by the radiation detector can be superimposed or overridden by the background radiation of the heat lamps. As a consequence, there results a very unfavorable signal (radiation emitted from the wafer) to the background (background radiation emitted from the heat lamps) ratio. This problem is increased in particular with low wafer temperatures, since the radiation emitted from the wafer rapidly decreases as the temperature decreases. Therefore, at low wafer temperatures the signal-to-background-ratio is also reduced. Below approximately 400° to 500° Celsius the wafer emits only a very low radiation, and furthermore below this temperature, in the case of a silicon wafer, the wafer is transparent for the heat radiation, so that the signal-to-background-ratio becomes even worse. For temperatures less than 400° Celsius, it is therefore generally not possible with the conventional process to determine the wafer temperature with a pyrometer.
To improve the signal-to-background-ratio with a pyrometer-based temperature measurement in RTP units, it is proposed in DE-A-40 12 614 to make the process chamber from an OH-containing quartz material. Such a quartz material has the characteristic of absorbing infrared light in the wavelength range of between 2.7 μm and 2.8 μm. Thus the wafer that is disposed in the process chamber is heated by a radiation, the spectrum of which has a gap between 2.7 μm and 2.8 μm. Provided in the quartz chamber is a sight window that is transparent in the aforementioned wavelength range, and through which a pyrometer is directed onto the wafer. The pyrometer now measures infrared radiation of the wavelength 2.7 μm coming from the wafer. Since the radiation in the wavelength of 2.7 μm emitted from the heat lamps cannot penetrate into the process chamber, the pyrometer measures only temperature radiation emitted from the wafer. With this method, the radiation intensity of the wafer can be well determined, and hence the radiation temperature can be determined. However, if the emissivity of the wafer deviates significantly from 1, which is the case in conventional practice, it is necessary to have an emissivity correction for the determination of the wafer temperature, or a calibration relative to the absolute wafer temperature must be undertaken.
Thus, with known methods the temperature radiation of the wafer can be well determined. However, in practice for the temperature determination of the wafer it is also necessary to know the reflectivity and transmissivity thereof at the wavelength 2.7 μm. This reduces the effort and expense of any calibration process.
The method known from DE-A-199 05 524 for the determination of the reflectivity, transmissivity and the emissivity resulting therefrom, and which utilizes a characteristic modulation to measure reflected radiation and radiation passing through the wafer, can be realized at temperatures below 400° to 500° only with very high expense for apparatus, since at these temperatures the signal-to-background ratio is very small.
It is therefore an object of the present invention, in a simple and economical manner, to provide a pyrometer-based temperature measurement of substrates that enables an exact temperature measurement, even at low temperatures.